With circuits with integrated optics the frequent need exists to exclude photosensitive areas, which do not belong to the receiving elements for optical radiation, targeted from the irradiation with light. This does not concern the shading effect only, on the contrary it is essential to absorb the light completely so that a suppression of the reflectance as completely as possible results and optical systems are not restricted in their efficiency by reflected or back scattered light. If e.g. an image sensor, which is used in combination with a lens, shows reflectances in the wavelength range used, then this leads to a reduction of the contrast range as with a lens in which the lenses used are not high-quality antireflection coated.
The application of metal layers and layers of black paint corresponds to the state of the art for light screening. In the patent specification EP 0590598 A1 a semiconductor device is described, which includes photodiodes, an integrated circuit surrounding the same and having several levels of metallization, which are electrically insulated by insulating layers from each other, and bonding sites on a chip, where the chip is provided with one or several light shielding layers of metal in the areas outside of the optical windows.
In the Japanese document JP 62 076570 A, a photo sensor with light shielding layers out of aluminum is described. Very similar applications of metal layers for the protection against incident light are in the documents DE 37 05 173 C2, EP 450,496 B1, EP 495,503 B1, JP 02 000376 A, JP 60 170255 A. According to all these publications, metal layers in different arrangements are used for shielding light. With use metal layers for shielding light, e.g. in combination with the CMOS technology, the technological modification is small, however, there exists the significant disadvantage of the high reflectance. The process steps used for the preparation of the metal layers to be efficiently integrated into the production run for the preparation of the circuit elements, e.g. of transistors, however, the reflectances caused by these metal layers lead again to the occurrence of radiation, which can again arrive by scattering at the photosensitive circuit areas to be protected, etc. Another disadvantage is the loss of a wiring plane for wiring in the area which is to be shielded from the light.
With use of black paint, as for example described in EP 816,899 B1, the high remaining transmission is of disadvantage. Paint does not shield by 100%, but still transmits a portion of the light. Another disadvantage exists in the deposition of the paint on the surface of an integrated element. Because of the larger distance to the photosensitive area of the element, which is typically located in the semiconductor plane and thus is remote from the paint by the several levels of metallization and the passivation layer, the edges cannot be defined very precisely. The large distance reduces the overlay accuracy with the structures in the device and leads to the shading of optical active areas and/or, with oblique light incidence, to the irradiation into areas which are to be blocked. This effect becomes stronger the larger the distance is of the black paint to the photosensitive range of the element, the smaller the photosensitive range (the surface of the element) is and the more oblique the light is coupled in. Furthermore, it is not possible to obtain a planar surface since the black paint has to be structured and high edges are developed thereby. Directly applying other layers or components on such a surface is not possible in a simple way.